PORTLAND, Ore. (AP) -- The days when uncaged cats, monkeys, snakes and chickens can ride TriMet buses and trains as "service animals" (it happens more than you might think) are numbered.

But guide horses? Well, those will be good to go under new conduct rules Oregon's largest transit agency is expected to approve next week.

Yep. You read that right. Guide horses.

TriMet says it's just following the lead of the Americans with Disabilities Act, which will narrow the definition of service animals in March to dogs and miniature horses trained to be guides for people who are blind or deaf.

Already frustrated with riders who appear to be taking advantage of the ADA to bring assorted pets on public transit, bus and MAX drivers are now worried that they'll be forced to haul around what they see as livestock.

"What's the definition of miniature?" said Willy Moore, a Line 38 bus driver. "Is Roy Rogers going to come on board with a little Shetland pony?"

Actually, trainers and advocates for the disabled say miniature horses are no taller than most guide dogs. They're also highly trainable, even when it comes to dropping, ahem, garden material.

What's more, TriMet and federal officials who administer the ADA say the stricter rules will clearly define what can be legitimately treated as a service animal.

That has been a problem in recent years. Federal authorities say they understand why everyone from bus drivers to supermarket managers have increasingly rolled their eyes at what people insist are service animals.

John Dineen, spokesman for the Northwest Americans with Disabilities Act Center in Seattle, said the list has strayed into the realm of the far-fetched, which isn't good for the law's image.

Dineen brought up the 2009 case of a southwest Washington resident who walked into a restaurant with his pet boa constrictor. The man claimed he needed the snake because it alerted him to pending seizures by giving him a hug.

"Animals such as reptiles and primates seem to have a tenuous link to service animals," Dineen said.

OK, so people with "guide snakes" are no longer protected. But why would someone choose Tiny Trigger over Rover?

Alexandra Kurland, a Delmar, N.Y., horse trainer and author of "Clicker Training for Your Horse," said guide horses have several advantages.

For starters, miniature horses usually live past 30. Kurland said horses also have excellent vision, are herd animals that do well in crowds and are considered less threatening than large dogs.

"In terms of actual skills of guiding, they are very suitable to the work," she said. "They tend to be mindful of their footing. Horses are very naturally attuned to stopping, and they're extremely alert to changes in elevation."

That's not to say people are abandoning their guide dogs en masse for horses.

Although some, including the North Carolina-based Guide Horse Foundation, consider horses the service animal of the future, they're still extremely rare in that role.

Kurland, who has trained only one, said there are two big drawbacks. One is cost. Since no guide horse schools have popped up, breeding and training miniature horses remains a highly specialized service, costing about $60,000 per animal.

TriMet gets at least one customer complaint a week about animals on buses and MAX. Operators say they have seen riders bring everything from an orangutan to birds on board, but never a horse.

Starting in March, TriMet riders would have to put all critters not approved as service animals in carriers. TriMet says guide horse owners will need to get advance approval.

Portland consistently rates high nationally for its dog-friendliness. But Jeff Guardalabene, a psychologist who takes TriMet from Northeast Portland to Forest Grove daily, said he is concerned about aggressive dogs that clearly aren't trained to be service animals.

Earlier this week, he tweeted a photo of a puppy, a rope around its neck, playfully biting at a MAX passenger.

"It's cute," Guardalabene said, "but I'm thinking, 'What might happen if a child runs by at the wrong time?' The fewer wild cards on trains and buses, the better, I think."

Two years ago, TriMet suspended the owner of a Rottweiler mix that attacked and killed a Pomeranian service dog on a bus. But under the ADA, the agency's options for turning away dogs they suspect as pets are limited.

Service dogs don't have to be licensed or certified for a specific task. They don't need a special collar or vest. The owners need only claim it's a service animal.

Legally, Dineen said, bus drivers can ask only two questions: "Is that a service animal?" and "What service does it perform?"

Moore said he might as well not even ask on his bus route.

"Anyone with a pet can go, 'Wink, wink, wink -- this is a service animal,'" he said. "It's pretty much don't ask, don't tell."